1. Field of the Invention
The present invention relates to an evaporated fuel control system in an internal combustion engine, which has a canister filled with an adsorbent for adsorbing an evaporated fuel in a fuel tank, a purge amount control means provided in a middle of a purge passage connecting an intake passage and the canister, and a fuel supply means.
2. Description of the Prior Art
An evaporated fuel control system is conventionally broadly known which prevents evaporated fuel generated in a fuel tank from being released into the atmosphere during stoppage of an engine. The system of such a type is designed such that evaporated fuel from the fuel tank is adsorbed by an adsorbent in the canister during the stoppage of the engine, and the evaporated fuel adsorbed by the adsorbent is released or purged into an intake system, for example, as disclosed in Japanese Patent Application,Laid-open No. 26362/87.
In the above prior art system, a duty control valve is provided in the purge passage between the canister and the intake passage and its duty ratio is gradually increased when it is shifted from 0% to 100%, thereby reducing the variation in air-fuel ratio during shifting from a non-purging condition to a full purging condition, thus providing an improvement in the nature of an exhaust gas.
Immediately after the start of purging, however, the evaporated fuel, that may be purged from the canister into the intake passage, shows a tendency in many cases that contains hydrocarbon in a higher concentration as the purge amount integration value is reduced. In such a case, particularly at a low temperature at which a catalyst in an exhaust system is not in an activated condition, the hydrocarbon may exert an adverse influence on the nature of the exhaust gas in some cases. No consideration has been given to this respect in the above prior art control system. Therefore, the prior art does not assure a sufficient improvement in the nature of the exhaust gas.